When drilling subterranean wells such as, for example, oil or gas wells, the rotary method is commonly employed. The rotary drilling method utilizes a bit attached to a drill stem, and a drilling fluid or "mud" which is circulated throught the drill stem to the bottom of the borehole where is is ejected through small openings in the drill bit. The fluid is then returned to the surface through the annular space between the drill stem and the borehole wall, or casing if one has been installed. Upon reaching the surface, the drilling fluid or "mud" is ordinarily treated to remove cuttings obtained from the borehole, and is then recirculated.
Drilling fluids serve many functions, and should therefore possess a number of desirable physical and rheological properties. For example, the viscosity of a drilling fluid should be sufficient to permit it to effectively transport bit cuttings from the bottom of the borehole to the surface for removal. A drilling fluid should also pevent excessive amounts of fluid from flowing from the borehole into surrounding formations by depositing on the wall of the hole a thin but substantially impervious filter cake. In addition, a drilling fluid should be able to hold solids in suspension, preventingtheir return to the bottom of the hole when the circulation is reduced or temporarily interrupted. This property can be obtained by utilizing additives which will impart a gel structure to the drilling fluid to increase viscosities. The gel structure, however, is preferably such that cuttings can be removed from the drilling fluid by passing the fluid through filtration equipment such as a shale shaker and/or sand cyclones prior to recirculating the fluid to the drill bit. A drilling fluid must also exert pressure on the surrounding formations, thus preventing possible collapse of the borehole or influx of highly pressurized oil or gas in the formation. Finally, a drilling fluid should serve as a lubricating and cooling agent for the drillstring and the bit.
Drilling of easily dispersible formations such as shales, marls and chalks often presents a problem in mud solids control. This problem and one method of controlling it are described in U.K. patent specification No. 1,256,538. Drilled solids from such formations tend to disintegrate in the drilling fluid while being transported to surface and the fines thus created are very difficult to remove. A build-up of fines is the consequence and leads to an increased viscosity of the drilling fluid and a decreased rate of penetration of the drilling operation. Ultimately mud dilution is required to recondition the mud or drilling fluid.